Recently, since there is a concern that fossil fuels are feared to be exhausted, alternative fuels are being developed. In particular, bioethanol derived from biomass has attracted attention. This is because biomass is a renewable resource, which exists in large amounts on the earth, and can be used without increasing carbon dioxide in the air (carbon neutral) so as to contribute to the prevention of global warming.
However, bioethanol is produced mainly from corns or sugarcanes nowadays, and therefore, there is a problem in that bioethanol competes with food. Therefore, in the future, the production of bioethanol from lignocellulosic biomass such as rice straw, wheat straw, waste wood or the like that does not compete with food will be required.
The lignocellulosic biomass includes mainly three components that are cellulose, hemicellulose and lignin. Cellulose is decomposed (saccharified) into glucoses by hydrolysis, and thus, can be used in ethanol fermentation by a yeast Saccharomyces cerevisiae that can utilize glucose.
Cellulose hydrolases such as cellulase are generally used to hydrolyze cellulose, and a pretreatment for separating cellulose from biomass and exposing it is performed prior to the enzymatic reaction in order to facilitate the enzyme reacting with cellulose. Conventionally, a hydrothermal decomposition method, an acid treatment method and an alkaline treatment method have been known as the pretreatment. A method of using dilute acid in a high temperature (200° C. or higher) and a method of using concentrated sulfuric acid or the like are known as the acid treatment method. However, since cellulose is partially decomposed under severe conditions by the hydrothermal decomposition method or by the acid treatment method, there is a problem in that over-decomposed products (by-products) are produced, glucose yield (saccharification rate) is low, and substances that inhibit the ethanol fermentation can be also produced.
Lignocellulosic biomass pretreatment products are subjected to an enzymatic treatment. In the enzymatic treatment, a cellulose component and a hemicellulose component included in the pretreatment products are hydrolyzed to produce oligosaccharides and monosaccharides. However, since the potency of the commercially-available enzyme used in the saccharification is low and thus a large amount of the enzyme is needed for sufficient saccharification, there is a problem in that the cost is increased.
On the other hand, attempts have been made in which the yeast Saccharomyces cerevisiae or the like that cannot originally utilize cellulose, hemicellulose and the like is modified using a bioengineering technique to produce ethanol directly from the biomass pretreatment products. A cell surface display technique is used as such a bioengineering technique. For example, a yeast displaying the group of enzymes that hydrolyze cellulose, that is, endoglucanase, cellobiohydrolase, β-glucosidase and the like, on its surface is produced using the cell surface displaying technique (Patent Document 1).